This invention relates to an apparatus for performing computed tomography comprising a radiation source for projecting a number of radiation beams in various directions from the outer periphery of a sectional portion of a subject to be displayed in parallel with both sides of the sectional portion, a detecting system for detecting radiation beams having penetrated the sectional portion and producing projection data signals corresponding to the strength of the respective radiation beams, a projection data memory for storing the signals in sets each corresponding to a fixed number of beams parallel to a predetermined direction of projection, and a display unit for displaying an image in accordance with the distribution of the radiation absorption coefficients at multitudinous points of the sectional portion, as well as to a method for effecting tomographic analysis by using the above mentioned apparatus.
This general type of apparatus is already known. According to the principle of such apparatus, data produced by projection of the radiation absorption coefficients at various parts of the transmission path, i.e. projection data, are obtained from radiation beams penetrating the sectional portion of the subject in parallel therewith and in fixed directions of projection at varied angles, and then the projection data are filtered for the calculation of modified projection data. A modified reconstructed image is obtained by conducting back projection on the basis of these modified projection data, and is displayed on the display unit. The filtering operation is performed in order to remove fuzziness or blurring that may be caused if the image is constructed from the data back projected as they are. Already known are a number of methods in which an image is reconstructed from modified projection data. The methods of convolution integration and filtered-back projection among others are known as acceptable methods for image reconstruction. Being well-known, these methods are excluded from the description herein. With conventional reconstruction methods, the integral computation employed for the convolution operation, Fourier transform, inverse Fourier transform, and the aforesaid back projection is based on an integral interval covering the whole area of a plane that crosses the subject, and reconstruction of the image of the sectional portion of interest is done only by conducting these operations.
In displaying the interior of the subject, e.g. in diagnosing the interior of the body of a patient, however, the sectional portion hardly requires overall display, often requiring only partial display, that is, of a specified or partial region to be observed. Nevertheless, in the prior art methods, reconstruction of the image would be performed also for unnecessary portions, requiring much time for operation. Convention systems, in which image reconstruction is conducted by the aforesaid operations, have been considered incapable of reconstructing and displaying an image of a partial region.
An object of this invention is to provide a method and apparatus for tomographic analysis by means of penetrating radiation capable of reconstructing an image of a specified region or partial region in a sectional portion of a subject in a short time without substantially deteriorating its quality as well as of displaying such image by utilizing part of the projection data relating to the sectional portion.